Method and apparatus for selecting carriers

ABSTRACT

This invention relates to a method and apparatus for selecting carriers to be measured by a subscriber unit ( 301,303, 501 ). The subscriber unit ( 301, 303, 501 ) is served by a communication system and is operable to measure carriers in a frequency band, which typically belongs to a different communication system. The apparatus comprises a performance processor ( 329 ) for determining a measuring performance characteristic of the subscriber unit ( 301, 303, 501 ) related to the measurement performance of the subscriber unit ( 301, 303, 501 ) when measuring in the frequency band. The apparatus furthermore comprises a carrier subset selector ( 331 ) for selecting a subset of carriers in the frequency band to be measured by the subscriber unit. The subset of carriers is selected dependent on the measuring performance characteristic. The invention is applicable to dual 2nd and 3rd generation cellular communication systems such as GSM/UMTS.

This application is a 371 of PCT/EP03/00788 Jan. 27, 2003.

FIELD OF THE INVENTION

This invention relates to a method, apparatus and subscriber unit forselecting carriers to be measured by a subscriber unit served by acellular communication system.

BACKGROUND OF THE INVENTION

In a cellular communication system, each of the subscriber units(typically mobile stations, communication terminals, wireless devices,user equipment, remote terminals etc) communicates with a fixed basestation. Communication from the subscriber unit to the base station isknown as uplink, and communication from the base station to thesubscriber unit is known as downlink. The total coverage area of thesystem is divided into a number of separate areas or cells, eachpredominantly covered by a single base station. The cells are typicallygeographically distinct with an overlapping coverage area withneighbouring cells. FIG. 1 illustrates a cellular communication system100. In the system, a base station 101 communicates with a number ofsubscriber units 103 over radio channels 105. In the cellular system,the base station 101 covers users within a certain geographical area107, whereas other geographical areas 109, 111 are covered by other basestations 113, 115. Some overlap areas 117 can be covered by more thanone cell.

As a subscriber unit moves from the coverage area of one cell to thecoverage area of another cell, the communication link will change frombeing between the subscriber unit and the base station of the firstcell, to being between the subscriber unit and the base station of thesecond cell. This is known as a handover. Specifically, some cells maylie completely within the coverage of other larger cells.

All base stations are interconnected by a fixed network. This fixednetwork comprises communication lines, switches, interfaces to othercommunication networks and various controllers required for operatingthe network. A call from a subscriber unit is routed through the fixednetwork to the destination specific for this call. If the call isbetween two subscriber units of the same communication system, the callwill be routed through the fixed network to the base station of the cellin which the other subscriber unit currently is. A connection is thusestablished between the two serving cells through the fixed network.Alternatively, if the call is between a subscriber unit and a telephoneconnected to the Public Switched Telephone Network (PSTN) the call isrouted from the serving base station to the interface between thecellular mobile communication system and the PSTN. It is then routedfrom the interface to the telephone by the PSTN.

Traditional traffic in mobile cellular communication systems has beencircuit switched voice data where a permanent link is set up between thecommunicating parties. In the future, it is envisaged that datacommunication will increase substantially and typically the requirementsfor a remote terminal to transmit data will not be continuous, but willbe at irregular intervals. Consequently it is inefficient to have acontinuous link set up between users. Instead a significant increase inpacket based data traffic is expected, where the transmitting remoteterminal seeks to transmit the data in discrete data packets whennecessary. An example of a packet based system is the General PacketRadio Service (GPRS) introduced for the Global System for Mobilecommunication (GSM). Further details on data packet systems can be foundin ‘Understanding data communications: from fundamentals to networking,2nd ed.’, John Wiley publishers, author Gilbert Held, 1997, ISBN0-471-96820-X.

A cellular mobile communication system is allocated a frequency spectrumfor the radio communication between the subscriber units and the basestations. This spectrum must be shared between all subscriber unitssimultaneously using the system.

One method of sharing this spectrum is by a technique known as TimeDivision Multiple Access (TDMA). In a TDMA communication system, thefrequency spectrum is typically divided into a number of separatefrequency channels or carriers and for each of these carriers aplurality of subscriber units are served by being allocated distincttime intervals. Thus in the example of the IDMA communication systemGSM, the frequency spectrum is divided into 200 kHz frequency channelseach of which is divided into eight separate time slots. A subscriberunit is allocated a specific time slot on a given frequency channel forcommunication with the serving base station. Further details of GSM canbe found in ‘The GSM System for Mobile Communications’, Bay ForeignLanguage Books, authors Michel Mouly and Marie-Bernadette Pautet, 1992,ISBN 2950719007.

Another method of sharing cellular spectrum is by a technique known asCode Division Multiple Access (CDMA). In a Direct Sequence CDMA(DS-CDMA) communication system, the signals are multiplied by a highrate code whereby the signal is spread over a larger frequency spectrum.A narrowband signal is thus spread and transmitted as a wideband signal.At the receiver, the original narrowband signal is regenerated bymultiplication of the received signal with the same code used to spreadthe signal in the transmitter. A signal which has been spread by use ofa different code will not be de-spread by the receiver but will remain awide band signal. It will then be removed by filtering after thede-spreading operation. In the receiver, the majority of interferencefrom signals in the same frequency spectrum as the wanted signal canthus be removed by filtering. Consequently, a plurality of subscriberunits can be accommodated in the same wideband spectrum by allocatingdifferent codes for different subscriber units. Codes are chosen tominimise the interference caused between subscriber units typically bychoosing orthogonal codes when possible. A further description of CDMAcommunication systems can be found in ‘Spread Spectrum CDMA Systems forWireless Communications’, Glisic & Vucetic, Artech house Publishers,1997, ISBN 0-89006-858-5. Examples of CDMA cellular communicationsystems are IS 95 and the Universal Mobile Telecommunication System(UMTS).

Currently many cellular communication system operators are in theprocess of rolling out UMTS or other 3G communication systems. It istypically intended that these 3G systems will interoperate with the 2Gcommunication systems such as GSM. Specifically, it is expected thatUMTS communication systems will initially be rolled out in separateislands of coverage, and consequently most of the initial releasemobiles will be dual mode UMTS/GSM mobiles, in order to enable useoutside areas of UMTS coverage. Effective handover between UMTS and GSMis thus imperative when mobiles roam in and out of UMTS coverage.

UMTS and GSM standards are being written in a way to enableimplementation of two types of multi-RAT (Radio Access Technology)mobiles, namely single receiver and dual receiver mobiles. Dual receivermobiles can simultaneously receive both GSM and UMTS signals, so thesemobiles can quickly perform measurements necessary for inter-systemhandover during active calls. However, single receiver mobiles can onlymeasure other systems during idle periods, when the receiver of thesubscriber unit is not active in receiving a call. In order to permitsingle receiver subscriber units to measure during an active UMTS call acompressed mode of operation has been standardised. In this method, theinformation of the active call is transmitted at increased data rate,thereby freeing up a time interval during which GSM measurements can bemade. Single receiver mobiles in active GSM calls can perform UMTSmeasurements during GSM idle frames (the mobile synthesizer switchingtime does not allow UMTS measurements during idle GSM time slots). Thisresults in long inter-RAT synchronisation times, e.g. a single receivermobile in an active GSM call may require 1 second to synchronise to oneUMTS cell under good channel conditions and 5–16 seconds under poorchannel conditions. This is a highly unacceptable delay, which willresult in decreased handover performance and an increased number ofdropped calls.

Hence, the current schemes for measuring carriers result in unacceptabledelays and degraded handover performance and there is thus a need for animproved system for measuring carriers.

SUMMARY OF THE INVENTION

The invention seeks to provide an improvement in the measuring ofcarriers in a frequency band by selecting carriers in response to ameasuring performance characteristic of the subscriber unit.

Accordingly there is provided, a method of selecting carriers to bemeasured by a subscriber unit served by a cellular communication systemand operable to measure carriers in a frequency band, the methodcomprising the steps of: determining a measuring performancecharacteristic of the subscriber unit related to the measurementperformance of the subscriber unit when measuring carriers in thefrequency band; and selecting a subset of carriers in the frequency bandto be measured by the subscriber unit in response to the measuringperformance characteristic.

By selecting a subset of carriers in the frequency band to be measuredin response to a measuring performance characteristic of the subscriberunit, the number, order and priority of carriers can be optimised tomeet the capability of the subscriber unit. Hence, for subscriber unitshaving high measurement performance, demanding subsets of carriers canbe chosen, whereas for subscriber units with low measurementperformance, less demanding subsets of carriers can be chosen.

Preferably, the invention further comprises the steps of the subscriberunit measuring the carriers of the subset of carriers and determining ahandover candidate carrier from the subset of carriers.

According to a feature of the invention, the measuring performancecharacteristic is indicative of the time required for the subscriberunit to measure carriers in the frequency band. Thus the measurementperformance characteristic will directly relate to the capability of thesubscriber unit to perform measurements within the time constraintsimposed by the communication system.

According to another feature of the invention the measuring performancecharacteristic is indicative of the number of receivers in thesubscriber unit. Thus the carriers selected will be chosen to reflectwhether the subscriber unit has the ability to measure while receivingdata communication or not, thereby enabling advantage to be taken of theability of multiple receiver subscriber units, while not compromisingthe performance of single receiver subscriber units.

According to further features of the invention, the frequency band is afrequency band of a second communication system using different radioaccess technologies, preferably one communication system being a GSMcommunication system and the other a UMTS communication system. Hence, afast and efficient method of performing cross system measurementsoptimised for the measurement capability of individual dual mode GSM andUMTS subscriber units is provided.

According to a second aspect of the invention, there is provided anapparatus for selecting carriers to be measured by a subscriber unitserved by a cellular communication system and operable to measurecarriers in a frequency band, the method comprising: means fordetermining a measuring performance characteristic of the subscriberunit related to the measurement performance of the subscriber unit whenmeasuring carriers in the frequency band; and means for selecting asubset of carriers in the frequency band to be measured by thesubscriber unit in response to the measuring performance characteristic.

According to a third aspect of the invention, there is provided asubscriber unit operable to be served by a cellular communication systemand to measure carriers in a frequency band, the subscriber unitcomprising: means for determining a measuring performance characteristicof the subscriber unit related to the measurement performance of thesubscriber unit when measuring carriers in the frequency band; and meansfor selecting a subset of carriers in the frequency band to be measuredby the subscriber unit in response to the measuring performancecharacteristic; and means for measuring the carriers of the subset ofcarriers.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is described below, by way ofexample only, with reference to the Drawings, in which:

FIG. 1 is an illustration of a cellular communication system accordingto prior art;

FIG. 2 is an illustration of an island of coverage deployment of a UMTSsystem;

FIG. 3 is an illustration of a communication system in accordance withan embodiment of the invention;

FIG. 4 is an illustration of a flow chart of a method of selectingcarriers in accordance with an embodiment of the invention; and

FIG. 5 is an illustration of a subscriber unit in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The intention of most operators of 3^(rd) generation (3G) communicationsystems, such as UMTS, is to provide only limited coverage for 3Gservices. In order to compensate for the limited coverage, it isintended that existing 2^(nd) generation systems, such as GSM, will beused to provide coverage where there is no 3G coverage. This deploymentstrategy is known as island of coverage deployment and is illustrated inFIG. 2.

As FIG. 2 shows, full coverage is provided by a number of GSM cells 501,507, 511, 513. Within the coverage area of the GSM communication system,some areas are covered by a UMTS communication system by UMTS cells 503,505, 509. In the example given, the UMTS base stations (node Bs) areco-located with the GSM base stations (BTSs) and do not overlap otherUMTS cells. However in other deployment scenarios, UMTS cells may not beco-located with GSM cells and may be adjacent or over lapping. Asubscriber unit located within a serving UMTS cell 503 can be providedwith 3G services within this cell. However, as the subscriber unit movesacross the border of the UMTS cell, it will be necessary to handover toanother cell in order not to drop the call. As there are no UMTS cellsavailable for handover, the subscriber unit will handover to a GSM cell501, which in this case will become the GSM serving cell 501. Typicallythe GSM cell will not be able to provide the same services as the UMTScell and therefore a change of service may be associated with the 3G to2G handover, for example the data rate of the communication may besignificantly reduced. As the subscriber unit moves into a different GSMcell 507, a 2G to 2G handover will be effectuated, so that cell 507becomes the serving GSM cell. If the subscriber unit moves further intothe GSM cell 507, it may enter the UMTS cell 505. At this point it willbe desirable to hand over to the UMTS cell 505 in order to takeadvantage of the enhanced 3G services (such as for example the increaseddata rate of the initial UMTS cell 503).

Clearly it is preferable that 3G subscriber units are dual modesubscriber units having the capability to operate on both 2G and 3Gcommunication systems. Further, it is important that handovers both from2G to 3G and from 3G to 2G communication systems are efficient andspeedy in order to maximise the network capacity and minimise thedisruption to the user. Hence, intersystem handovers must be fast,accurate and reliable. In order to achieve efficient handovers, thesubscriber unit must perform a number of measurements on the carriers ofsuitable handover candidates. Also the subscriber unit must be able tomeasure carriers of the other communication system in order to determineif handover is preferred to the current or to another communicationsystem.

FIG. 3 is an illustration of a communication system in accordance withan embodiment of the invention.

FIG. 3 illustrates a dual receiver subscriber unit 301, a singlereceiver subscriber unit 303 and a base station 305 representing thefixed network. For clarity only the aspects of the communication systemnecessary for the description of the embodiment are included in thefigure. As is well known in the art, many other functions and circuitsnot specifically shown are required for a fully functioningcommunication system,.

In the embodiment of FIG. 3, both subscriber units 301, 303 are servedby the base station 305. Both subscriber units 301, 303 are continuouslymeasuring the carriers transmitted from other cells in order todetermine if a better serving cell is available, in which case ahandover process is initiated.

The dual receiver subscriber unit 301 comprises a data receiver 307operable to receive and demodulate the communication signals transmittedfrom the base station 305. The receiver is coupled to a data processor310, which processes the demodulated data signals as appropriate, forexample by translating the received data into a speech signal providedto the user as is well known in the art. In addition, the dual receiversubscriber unit 301 comprises a second receiver which is a measurementreceiver 309 operable to tune to the carriers of other base stations andperform measurements of these carriers. In its simplest form, thereceiver simply measures the signal strength of other carriers but inmore complex embodiments the measurement receiver may synchronise to theother cells and fully demodulate the signal in order to determine otherparameters, such as a bit error rate or a frame error rate. Themeasurement receiver 309 is connected to the measurement processor 311,which has the dual function of processing the measurements andcontrolling the measurement receiver 309. The measurement processor 311is in the preferred embodiment connected to a transmitter (not shown) ofthe dual receiver subscriber unit 301 in order to transmit measurementreports back to the base station 305.

The single receiver subscriber unit 303 comprises a single receiver,which is operable to perform the dual function of receiving thecommunication signal from the base station 305, as well as returning toperform measurements of carriers of other cells. Consequently, thereceiver is connected to both a data processor 315 for processing thecommunication signal and a measurement processor 317 for processing themeasurements and controlling the measurement function of the receiver.The measurement processor 317 is further connected to a transmitter (notshown) for transmitting a measurement report back to the base station.As the receiver of the single receiver subscriber unit 303 has a dualfunction, measurements of other cells can only be made when the receiveris idle i.e. when it is not engaged in receiving the communicationsignals from the base station 305.

The base station 305 comprises a receiver 319 and a transmitter 321coupled to the same antenna 325 through a duplexer 323. The receiver 319is operable to receive signals from the subscriber units 301,303 whetherthese comprise data traffic, control data or other information.Specifically, the receiver 319 receives control messages and measurementreports from the subscriber units 301,303. The receiver is connected tocircuitry required for the further processing of data and controlmessages (not shown) as is well known in the art.

In addition, the receiver 319 is connected to a handover controller 327,which receives the measurements from the subscriber units and inresponse to these determines whether handover is required for eachindividual subscriber unit 301, 303. If handover is required itdetermines which target cell is the preferred handover candidate. In thesimplest embodiment, a handover is executed when the quality of service(e.g. the bit error rate) increases above a given threshold, and thetarget cell is chosen as the cell reported having the highest signallevel. However, many alternative handover algorithms are well known inthe art and may be used in different embodiments.

The receiver 319 is further connected to a performance processor 329operable to determine a measuring performance characteristic of thesubscriber unit related to the measurement performance of the subscriberunit when measuring carriers in the frequency band.

The performance processor 329 is connected to a carrier subset selector331 operable to select a subset of carriers to be measured by thesubscriber unit. The carrier subset selector 331 selects the subset ofcarriers in response to the measuring performance characteristicdetermined by the performance processor 329.

FIG. 4 is an illustration of a flow chart of a method of selectingcarriers in accordance with an embodiment of the invention. The methodis suitable for the communication system of FIG. 2.

In step 401, a measurement performance characteristic of the subscriberunit is determined. In the preferred embodiment this is determined inthe fixed network, and in the specific example it is determined in thebase station 305 by the performance processor 329.

In the preferred embodiment, the measurement performance characteristicrelates to whether the subscriber unit is a single receiver subscriberunit 303 or a dual receiver subscriber unit 301. Thus the measurementperformance characteristic relates to whether the subscriber unit hasone or two receivers, and thus whether it can measure other cellcarriers continuously or only during idle periods. In other embodiments,the measurement performance characteristic may relate to othercharacteristics of the measurement process or subscriber unitmeasurement capability, such as for example the possible accuracy of themeasurements, the sensitivity of the measurement receiver, the speed atwhich measurements can be communicated to the network, the number ofdifferent carriers that can be measured, the extent of the frequencyband which can be measured etc. Hence, the measurement performancecharacteristic may relate to any characteristic related to themeasurement performance of the subscriber unit when measuring carriers.

In the preferred embodiment the measurement performance characteristicof a subscriber unit is determined from control messages reported by thesubscriber unit and indicating the number of receivers it comprises.

Thus in the preferred embodiment of a dual GSM/UMTS communicationsystem, the single/dual receiver capabilities of the subscriber unit canfor GSM be obtained from the standardised UE CAPABILITY message reportedas part of the UTRAN CLASSMARK CHANGE message, while in UMTS this iscontained in the UE CAPABILIY INFORMATION message. The UE CAPABILITYmessage contains the MEASUREMENT CAPABILITY IE, which states whetheruplink or downlink compressed mode is required for measurements fromUMTS to GSM for different GSM bands. The base station can deduce that ifdownlink compressed mode is required for GSM measurements, thesubscriber unit has a single receiver, while if downlink compressed modeis not required, the subscriber unit has a dual receiver. (Note that theinformation on uplink compressed mode should not be used, since evendual receiver subscriber units may require uplink compressed mode toperform measurements on the DCS1800 band, due to proximity of this bandwith the UMTS core frequency allocation).

In other embodiments, the measurement performance characteristic isdetermined directly from evaluation of the subscriber unit's performancewhen measuring. In one embodiment, it is measured how frequentlymeasurement updates are received from a subscriber unit, and themeasurement performance characteristic is chosen as the average timebetween receipt of measurement reports.

In one embodiment, the measurement performance characteristic isdetermined from the number of measurement reports received from thesubscriber unit within a given time interval. Specifically for a dualGSM/UMTS communication system, the base station can determine thecapability of the subscriber unit by observing how many neighbours arereported during GSM active mode—i.e. during normal call operation. Thiscan be compared to the number reported during call set-up, where thesubscriber unit is on an SDCCH channel and so has more time to take UMTSmeasurements. If a lower number of neighbours are reported in activemode than in call set-up mode, it indicates that the subscriber unit hasa low capability for making measurements. This may be related to asingle receiver implementation but may also relate to other reasons forlow measurement performance (e.g., associated with the measurementprioritisation process in the subscriber unit, the service that thesubscriber unit is using, the mode of attachment to the network (e.g.DCH, FACH, PCH etc), or a battery saving mode). This method thereforehas the advantage of enabling a dynamic update of the measurementperformance characteristic to be made thereby reflecting the currentmeasurement performance of the subscriber unit.

In step 403, the determined measurement performance characteristic isfed to the carrier subset selector 331. The carrier subset selector 331selects a subset of all the possible carriers for the subscriber unit tomeasure. Thus in the example of a subscriber unit operating on a UMTScommunication system and measuring on a GSM communication system, asubset of all carrier frequencies within the frequency band occupied bythe GSM communication system is chosen. In the preferred embodiment, thesize of the subset is dependent on the measurement performancecharacteristic. Thus if the subscriber unit is determined as comprisinga single receiver e.g. four carriers may be included in the subset,whereas if the subscriber unit is determined as having two receiverse.g. eight carriers may be included in the subset of carriers. Thesubset is chosen to comprise the carriers of the neighbour cellsconsidered most likely to be suitable targets for handovers, for exampleby being reported as having been received with the highest signal levelby the subscriber unit. Hence in the specific example, if the subscriberunit is a single receiver subscriber unit, the carriers of the fourcells having the highest signal level are included in the subset. If thesubscriber unit is a dual receiver subscriber unit, the carriers of theeight cells reported with the highest signal level are included in thesubset.

In other embodiments, a more continuous selection of carriers for thesubset can be employed. In the specific embodiment of the measurementperformance characteristic being an average time between measurements,the number of carriers included in the subset may simply be inverselyproportional to the time between measurements.

In the case of GSM carriers being measured, the selection of carriers ismade by selecting the appropriate carrier frequencies to be measured.However, in other communication systems such as for example UMTS,different carriers may be at the same frequency but separated by othermeans. In this case carriers are selected as appropriate for thecharacteristics of that communication system. For example when measuringon a UMTS system, different carriers will typically use the samefrequency spectrum but have different spreading codes. In this case,selection of carriers is achieved by selecting the appropriate spreadingcodes of the carriers to be measured.

In step 405, identification of the selected subset of carriers is fed tothe transmitter 321 and transmitted to the appropriate subscriber unit.The identification may be by any suitable means, such as by identifyingthe specific frequency of each carrier, the spreading code of eachcarrier or any other method conveying the information of which carriersare included in the subset. Specifically, in the case of GSM and UMTScommunication systems the identification of the subset may becommunicated as a neighbour list. The neighbour list may contain cellsfrom the same, from different or from a mixture of communicationsystems.

The relevant subscriber unit receives the identification of the subsetfrom the base station 325, and in step 407, it proceeds to measure thecarriers identified to the subscriber unit. Thus the identificationmessage is received by the receiver 309 or 313 and decoded by the dataprocessor 307, 315, which feeds the identification to the measurementprocessor 311, 317. The measurement processor subsequently instigatesthe measurement process. If the subscriber unit is a single receiversubscriber unit 303, it will, in the specific example given above,receive identification of four carriers that it will begin to measure.If the subscriber unit is a dual receiver subscriber unit 301, it willreceive identification of eight carriers that it will begin to measure.

Once the measurement process is completed, the subscriber unit transmitsa measurement report to the base station 305. The measurement report isfed to the handover controller 327, which in step 411 determines if ahandover is required or desired. If so, the handover controller 327proceeds to determine the target handover candidate in step 413 andotherwise the process continues at step 407. If handover is required thehandover controller 327 instigates the handover in step 415 bytransmitting a handover command to the subscriber unit. It will be clearthat any known method or algorithm of determining a handover candidateor the requirement for a handover may be used without detracting fromthe invention.

FIG. 4 illustrates the preferred embodiment but it is within thecontemplation of the invention that any suitable order, interaction orfrequency of the different function steps can be used, includingperforming some function steps independently and/or in parallel withother function steps. For example, the determination of the measurementperformance characteristic may be performed only once during manufactureof the subscriber unit, whenever the subscriber unit registers on anetwork, following a handover or when requested by the base station.Similarly, the selection of carriers in the subset may for example beperformed continuously, following handovers or whenever a new call isinitiated.

By treating subscriber units differently according to the measurementperformance characteristic, improved network performance is achieved.Specifically, requesting too many measurements from receivers with lowmeasurement performance will amongst other things lead to increasedpower consumption, increased delay in measurements, excessively highresource demand of the measurement process and less resource for userdata communication. Conversely, subscriber units having bettermeasurement performance may be able to handle many measurements andlimiting the measurements taken will result in fewer carriers beingmeasured leading to reduced information being gathered about the radioenvironment of the specific subscriber unit. If handovers are based onthese measurements, an increase in sub optimal handovers and droppedcalls will result, leading to reduced system capacity. However, byselecting carriers to be measured in accordance with the measurementperformance of the subscriber unit, it is possible to optimise themeasurement process for each individual subscriber unit in accordancewith it's measurement capability, and thereby improve performance of thecommunication system(s).

Specifically for a combined UMTS/GSM system, requesting measurement onGSM too frequently from single receiver subscriber units will degradethe quality of service available to the user, the power consumption ofthe subscriber unit and the network capacity will be adversely affected.Conversely, if a similar level of measurements is requested from a dualreceiver subscriber unit the only impact is to slightly reduce UEbattery life.

In another embodiment, the determination of the measurement performancecharacteristic and selection of the subset of carriers is performedindependently by the subscriber unit. FIG. 5 illustrates a dual receiversubscriber unit in accordance with this embodiment of the invention.

The dual receiver subscriber unit 501 comprises a data receiver 503operable to receive and demodulate the communication signals transmittedfrom the base station (not shown). The data receiver 503 is connected toa data processor (not shown) for processing the communication signal asis well known in the art. In addition, the shown subscriber unit 501comprises an optional measurement receiver 505, whereas other subscriberunits of the communication system do not comprise this measurementreceiver 505. The measurement receiver 505 is operable to measurecarriers in a frequency band. For subscriber units not comprising theoptional measurement receiver 505, the data receiver 503 is operable toretune to measure carriers in the frequency band. Both the data receiver503 and the measurement receiver 505 are coupled to a handover processor507, which controls the handover functions of the subscriber unit. Thehandover processor is in turn connected to a performance processor 509for determining a measuring performance characteristic of the subscriberunit related to the measurement performance of the subscriber unit, whenmeasuring carriers in a frequency band. The performance processor 509 isconnected to a carrier subset selector 511 operable to select a subsetof carriers in the frequency band to be measured.

In this embodiment, the base station transmits a list of potentialneighbours to all subscriber units. The data receiver 503 of thesubscriber unit 501 receives the neighbour list and passes theinformation to the handover processor 507, which again feeds theinformation to the carrier subset selector 511. In addition, thehandover processor 507 controls the performance processor 509 todetermine a measurement characteristic of the subscriber unit. Thischaracteristic is fed to the carrier subset selector 511, which inresponse to the measurement performance characteristic selects a subsetof carriers from the neighbour list. Again, the neighbour list maycontain cells from the same, a different or from a mixture ofcommunication systems. Information of the selected subset is fed to thehandover processor 507, which controls the measurement receiver tomeasure the selected carriers. If the subscriber unit 501 only comprisesa single receiver 503, the handover processor 507 controls the datareceiver 503 to measure the selected carriers during idle times of thereceiver. Based on the measurements, the handover processor maydetermine if a handover is required and if so identify a target cell. Inother embodiments, the handover processor 507 transmits the measurementreport to the fixed network, and a handover processor in the fixednetwork determines handovers based on the measurements reports.

The performance processor 509 may simply determine the measurementperformance characteristic as whether the receiver has one or tworeceivers. In this case, it may be implemented as simply as a permanententry in a memory location of the subscriber unit, and thus themeasurement performance characteristic may only be determined onceduring manufacture of the subscriber unit. Alternatively, themeasurement performance characteristic can be determined continuouslyand reflect the dynamic chances of the measurement performance of thesubscriber unit. In this case the performance processor 509 interactswith other circuitry of the subscriber unit to determine how efficientthe measurement process currently is. The determination is preferably bydetermining the time delay or frequency of the measurements in thecurrent condition or configuration of the subscriber unit.Alternatively, the measurement capabilities of the subscriber unit maybe improved by it connecting with another device, such as anothersubscriber unit or and independent measuring device. Ea this embodiment,the measurement performance characteristic is dynamically updated as thesubscriber unit connects and disconnects to external devices.

In the simple embodiment of the measurement performance characteristicsimply relating to whether the subscriber unit has one or two receivers,the neighbour list passed to the carrier subset selector 511 can simplyconsist in a list of frequencies wherein some are intended to memeasured in all circumstances and others are only to be measured if thesubscriber unit comprises two receivers. Thus the neighbour listtransmitted by the base station may have the following structure:

Carrier Identity Measurement Requirement Carrier no. 45 All receiversCarrier no. 124 Dual receiver subscriber unit only Carrier no. 4 Allreceivers Carrier no. 53 All receivers Carrier no. 97 Dual receiversubscriber unit only Carrier no. 110 Dual receiver subscriber unit onlyCarrier no. 43 All receivers

In this example, a dual receiver subscriber unit will control themeasurement receiver to measure carriers 45, 124, 4, 53, 97, 110, 43 and97 whereas a single receiver will control the data receiver to measurecarriers 45, 4, 53 and 43.

In this embodiment, the measurement requirements for each carrier isdetermined by the fixed network on a cell by cell basis dependent on thelikelihood of a given carrier being the optimal handover candidate forsubscriber units within the cell. Thus, the carriers of neighbour cellsimmediately adjacent the serving cell may be selected for all cellswhereas more distant neighbour cells are only required to be measured bydual receiver subscriber units.

In more advanced embodiments a neighbour list provided does not relatedirectly to the measurement performance of the subscriber unit, but isordered in terms of priorities of the carriers. Hence, in thisembodiment the neighbour list transmitted by the base station may forexample be as shown below:

Carrier Identity Priority Carrier no. 45 Highest Carrier no. 124 HighestCarrier no. 4 High Carrier no. 53 Medium Carrier no. 97 Medium Carrierno. 110 Low Carrier no. 43 Low Carrier no. 97 Low

In this example, the carrier subset selector 511 selects carriers to bemeasured in order of their reported priority. Thus if the performanceprocessor 509 determines that the subscriber unit is currently capableof measuring three carriers within a suitable time interval, carriers45,124 and 4 are chosen for the subset. If the measurement performancecharacteristic currently indicates that six carriers can be measured,carrier no 45, 124, 4, 53, 97 and one of 110, 43 and 97 are chosen.

In a more sophisticated embodiment, the received neighbour list issimply a list of carriers ordered in terms of their relative priority(e.g. the likelihood of being the preferred handover candidate), and thesubscriber unit determines the measurement performance characteristic asa specific number of carriers which can be measured and reported withinthe time constraints imposed by the system. In this embodiment, thecarrier subset selector 511 simply chooses the specific number ofcarriers in order of their priority.

In yet another embodiment, the neighbour list transmitted from the basestation does not comprise any information related to the measurementperformance of the subscriber unit. Rather the subscriber unit in thisembodiment occasionally measures all carriers of the neighbour list, andbased on these occasional measurements the handover processor 507 ordersthe carriers of the neighbour list in order of measured signal strength.This ordered list is passed to the carrier subset selector 511, whichselects the strongest carriers to be measured for handoverdetermination. The number of carriers selected from the ordered listdepends the measurement performance characteristic.

The above description has focused on embodiments wherein measurementsare made for the purpose of determining handover candidates, but it iswithin the contemplation of the invention that it is suitable for anymeasurements benefiting from a selection of carriers in response to ameasurement performance irregardless of the purpose or further use ofthese measurements.

The components and functionality described may be implemented in anysuitable manner to provide suitable apparatus. Specifically, thecomponents may consist of a single discrete entity, may be distributedover a plurality of entities and/or may be formed by adapting existingparts or components. As such the required adaptation may be implementedin the form of processor-implementable instructions stored on a storagemedium such as floppy disk, compact disc, PROM, RAM or any combinationof these or other storage media. Furthermore the functionality may beimplemented in the form of hardware, software or any combination ofthese. As such it will be clear that the handover processor, performanceprocessor and carrier subset selector of the preferred embodiment areshown as part of the base station or subscriber unit as an example only,and that these may be implemented in other elements of the network, suchas in the Base Station Controller (BSC) for GSM and the Radio NetworkController (RNC) for UMTS.

It will be understood that the invention tends to provide a number ofadvantages including:

-   -   optimisation of a carrier measurement procedure for each        individual subscriber unit dependent on the measurement        performance of that subscriber unit.    -   allowing optimal trade off for each individual subscriber unit        between measurement disadvantages and advantages.    -   specifically, for handover measurements, a trade off between the        number of potential handover cells investigated and the speed of        a handover can be optimised for each individual subscriber unit.        This results in faster and more accurate handovers leading to        improved quality of service and increased system capacity.    -   reduced power consumption of subscriber units by allowing        measurements to dependent on the measurement performance.

1. A method of selecting carriers to be measured by a subscriber unitserved by a cellular communication system and having a number ofreceivers operable to measure carriers in a frequency band, the methodcomprising the steps of: Dynamically determining a frequency ofmeasurements updates when measuring carriers in the frequency band;wherein the frequency of measurements updates is indicative of theamount of time required by the subscriber unit to measure the carriersand wherein the amount of time required to measure the carriers isdependent on how many receivers a subscriber unit have; and Dynamicallyselecting a subset of carriers in the frequency band to be measured bythe subscriber unit based on the frequency of measurement updates whichis dependent on the frequency of measurement updates, which in turn isbased on the number of receivers in a subscriber unit.
 2. A method ofselecting carriers as claimed in claim 1 further comprising the step oftransmitting identification of the subset of carriers to the subscriberunit.
 3. A method of selecting carriers as claimed in claim 1 furthercomprising the step of the subscriber unit measuring the carriers of thesubset of carriers.
 4. A method of selecting carriers as claimed inclaim 1 wherein the frequency of measurement updates is indicative ofthe number of receivers in the subscriber unit.
 5. A method of selectingcarriers as claimed in claim 1 wherein the frequency band is a frequencyband of a second communication system.
 6. A method of selecting carriersas claimed in claim 1 wherein the cellular communication system and thesecond communication system use different radio access technologies. 7.A method of selecting carriers as claimed in claim 6 wherein thecellular communication system is a GSM communication system and thesecond communication system is a UMTS communication system.
 8. A methodof selecting carriers as claimed in claim 6 wherein the cellularcommunication system is a UMLTS communication system and the secondcommunication system is a GSM communication system.
 9. A method ofselecting carriers as claimed in claim 6 wherein an update frequencycapability of the subscriber unit is determined from a subscriber unitcapability report indicating if compressed mode is required to performmeasurements on a GSM communication system.
 10. A method of selectingcarriers as claimed in claim 1 wherein the size of the subset ofcarriers is dependent on an update frequency capability of thesubscriber unit.
 11. A method of selecting carriers as claimed in claim1 wherein the subset of carriers is an ordered subset of carriers andthe order of carriers in the subset of carriers is dependent on anupdate frequency capability of the subscriber unit.
 12. A method ofselecting carriers as claimed in claim 1 further comprising the step ofdetermining a handover candidate carrier from the subset of carriers.13. A method of selecting carriers as claimed in claim 1 wherein thefrequency of measurement updates is determined from the number ofmeasurement reports reported from the subscriber unit within a giventime interval.
 14. An apparatus for selecting carriers to be measured bya subscriber unit served by a cellular communication system and having anumber of receivers operable to measure carriers in a frequency band,the method comprising the steps of: Means for dynamically determining afrequency of measurements updates when measuring carriers in thefrequency band; wherein the frequency of measurements updates isindicative of the amount of time required by the subscriber unit tomeasure the carriers and wherein the amount of time required to measurethe carriers is dependent on how many receivers a subscriber unit have;and Means for dynamically selecting a subset of carriers in thefrequency band to be measured by the subscriber unit based on thefrequency of measurement updates which is dependent on the frequency ofmeasurement updates, which in turn is based on the number of receiversin a subscriber unit.
 15. An apparatus as claimed in claim 14 furthercomprising means for transmitting identification of the subset ofcarriers to the subscriber unit.
 16. An apparatus as claimed in claim 14wherein the frequency of measurement updates is indicative of the numberof receivers in the subscriber unit.
 17. An apparatus as claimed inclaim 14 wherein the frequency band is a frequency band of a secondcommunication system.
 18. An apparatus as claimed in claim 17 whereinthe cellular communication system and the second communication systemuse different radio access technologies.
 19. An apparatus as claimed inclaim 18 wherein the cellular communication system is a GSMcommunication system and the second communication system is a UMTScommunication system.
 20. An apparatus as claimed in claim 18 whereinthe cellular communication system is a UMTS communication system and thesecond communication system is a GSM communication system.
 21. Anapparatus as claimed in claim 18 wherein the means for determining isoperable to determine an update frequency capability of the subscriberunit from a subscriber unit capability report indicating if compressedmode is required to perform measurements on a GSM communication system.22. An apparatus as claimed in claim 14 further comprising means fordetermining a handover candidate carrier from the subset of carriers.23. An apparatus as claimed in claim 14 wherein the means fordetermining is operable to determine the frequency of measurementupdates from the number of measurement reports reported from thesubscriber unit within a given time interval.
 24. A cellularcommunication system comprising the apparatus of claim
 14. 25. Asubscriber unit for selecting carriers to be measured by a subscriberunit served by a cellular communication system and having a number ofreceivers operable to measure carriers in a frequency band, the methodcomprising the steps of: means for dynamically determining a frequencyof measurements updates when measuring carriers in the frequency band;wherein the frequency of measurements updates is indicative of theamount of time required by the subscriber unit to measure the carriersand wherein the amount of time required to measure the carriers isdependent on how many receivers a subscriber unit have; and means fordynamically selecting a subset of carriers in the frequency band to bemeasured by the subscriber unit based on the frequency of measurementupdates which is dependent on the frequency of measurement updates,which in turn is based on the number of receivers in a subscriber unit.26. A subscriber unit as claimed in claim 25 wherein the frequency ofmeasurement updates is indicative of the number of receivers in thesubscriber unit.
 27. A subscriber unit as claimed in claim 25 whereinthe frequency band is a frequency band of a second communication system.28. A subscriber unit as claimed in claim 25 wherein the cellularcommunication system and the second communication system use differentradio access technologies.
 29. A subscriber unit as claimed in claim 28wherein the cellular communication system is a GSM communication systemand the second communication system is a UMTS communication system. 30.A subscriber unit as claimed in claim 28 wherein the cellularcommunication system is a UMTS communication system and the secondcommunication system is a GSM communication system.
 31. A subscriberunit as claimed in claim 25 further comprising means for determining ahandover candidate carrier from the subset of carriers.